Dave Plowman (News) wrote:
In article ,
Stuart Noble wrote:
Pack between the frame and window with small wedges at your fixing
points so you have an at least 5mm gap all round and don't tighten the
screws so as to distort the window. Then fill the gap with silicone
sealer.

Or use expanding foam, which holds the window securely once it's set.
You can then do the fixings without fear of the frame moving about

Absolutely - had a brain fart there.

Know what you mean